1. Field of the Invention
The present invention relates to a multiple discharge lamp lighting apparatus, and particularly to a multiple discharge lamp lighting apparatus used as a backlight to light a plurality of discharge lamps for a liquid crystal display device.
2. Description of the Related Art
A discharge lamp, for example, a cold cathode lamp, is extensively used as a light source of a backlight for a liquid crystal display (LCD) device. Recently, an LCD device as typified by, for example, a display device for an LCD television, is coming out with a higher brightness and a larger size, and accordingly a multiple lamp backlight with a plurality of discharge lamps is heavily used as a lighting source and at the same time the length of the discharge lamp is increased.
Generally, a high voltage of high frequency is required for lighting a discharge lamp, and therefore a discharge lamp lighting apparatus includes an inverter means to convert a DC voltage into a high-frequency AC voltage, and a step-up transformer to boost the voltage. The primary side of the transformer is driven by the inverter means, and thereby a high high-frequency voltage is generated at the secondary side of the transformer and applied to the discharge lamp, thus the discharge lamp is lit.
In such a discharge lamp lighting apparatus as described above, the following problems are faced when the length of the discharge lamp is increased. If the length of the discharge lamp is increased, the voltage for lighting the discharge lamp must be also increased thus requiring a sufficient withstand voltage for the transformer, which results in difficulties with downsizing. Also, in such a discharge lamp lighting apparatus, usually one electrode of the discharge lamp is grounded together with one terminal of the secondary winding of the transformer, and therefore the potential at an ungrounded electrode fluctuates largely to the ground potential when the discharge lamp is turned on. As a result, a large luminance gradient is caused, especially for a long discharge lamp, with respect to the longitudinal direction of the discharge lamp thus impairing the luminance quality.
In order to overcome the problems described above, a discharge lamp lighting apparatus is disclosed which includes a circuitry as shown in FIG. 8 (refer to, for example, Japanese Utility Model Application Laid-Open No. H5-90897). Referring to FIG. 8, a discharge lamp lighting apparatus 100 includes a first oscillation transformer 121, a second oscillation transformer 125, and oscillation circuits 122 and 126 to drive the first and second oscillation transformers 121 and 125, respectively. One terminals of respective secondary windings 121s and 125s of the first and second oscillation transformers 121 and 125 are grounded, and the other terminals thereof are connected via respective ballast capacitors 128 to both electrodes of a discharge lamp 127. And, the discharge lamp lighting apparatus 100 is structured such that voltages phased opposite to each other are generated respectively at the terminals of the secondary windings 121s and 125s connected to the discharge lamp 127.
In the discharge lamp lighting apparatus 100 adapted to light the discharge lamp 127 by the two oscillation transformers 121 and 125, each of voltages generated at the secondary windings 121s and 125s of the two oscillation transformers 121 and 125 can be reduced by half compared with a discharge lamp lighting apparatus adapted to light a discharge lamp by one transformer, thus making it easy to downsize the transformer, and at the same time the potentials at the both electrodes of the discharge lamp 127 fluctuate equally about the ground potential thus reducing the luminance gradient with respect to the longitudinal direction.
However, the discharge lamp lighting apparatus 100 requires transformers in a number double the number of the discharge lamps to be lit, thus causing a cost increase.